In the fields of optical fiber communications, optical interconnection, optical signal processing and so on, an optical modulator that converts an electrical signal into an optical signal, an optical switch that controls a path and intensity of an optical signal, a variable optical attenuator, a tunable filter and the like are important functions. Such functions can be implemented by a planar optical waveguide circuit. For example, a Mach-Zehnder interferometer, a ring resonator or the like is made with a planar optical waveguide. Then, a refractive index in the interferometer or the resonator is controlled to change the state of interference or resonance. Consequently, it is known that it is possible to control the intensity or propagation direction of output light and thereby implement the-above functions.
For example, for transmission with a bit rate of 10 Gbps or more in long-distance optical fiber communications, a Mach-Zehnder interferometer type optical modulator using a lithium niobate optical waveguide is widely used. However, because the Mach-Zehnder interferometer type optical modulator using the lithium niobate optical waveguide has an element size of as large as several centimeters and requires a drive voltage of as high as several volts, there are issues such as that a dedicated driver circuit is necessary, integration with another optical element is difficult, and costs are high.
Recently, for improvement on the above issues regarding the lithium niobate optical modulator, development and commercialization of an optical modulator using a silicon optical waveguide are in progress (Patent Literatures 1 to 3). The silicon optical waveguide has a stronger action to confine light inside the waveguide than the lithium niobate optical waveguide. It thus has features such as capability to reduce an element size, to lower a drive voltage, to provide integration with another optical element or an electronic circuit, and to realize higher productivity and lower costs with use of LSI manufacturing resources.
As an example of the silicon optical modulator, FIG. 9 shows a schematic cross sectional structure of the optical modulator disclosed in FIG. 9 of Patent Literature 1. FIG. 9 shows an optical waveguide in a refractive index control part of the optical modulator. The cross sectional structure of the optical waveguide includes a silicon substrate 10, a buried oxide layer 11, a relatively thin submicron surface silicon layer 12, a gate dielectric layer 13, a relatively thin polysilicon gate layer 14, and a superposed dielectric layer 19. A part of the surface silicon layer 12 and a part of the polysilicon gate layer 14 are arranged to overlap with the gate dielectric layer 13 interposed therebetween. The surface silicon layer 12 is doped into p-type and the polysilicon gate layer 14 is doped into n-type, so that they make a MOS structure. Regions 212 and 214 are respectively formed in the surface silicon layer 12 and the polysilicon gate layer 14. The regions 212 and 214 are high concentration doped regions which are doped with a high concentration to establish electrical connection with the outside. The regions 212 and 214 are respectively placed at positions apart from the area where the surface silicon layer 12 and the polysilicon gate layer 14 overlap. Further, a region of the surface silicon layer 12 near the gate dielectric layer 13 is a region 112, and a region of the polysilicon gate layer 14 near the gate dielectric layer 13 is a region 114. The region 112 and the region 114 serve as carrier modulation regions where the carrier density varies.
The refractive index of the surface silicon layer 12 and the polysilicon gate layer 14 is about 3.5. Further, the refractive index of the buried oxide layer 11 and the superposed dielectric layer (typically, a silicon oxide film is used) 19 is about 1.45. Thus, light is confined in a region 101 near the part where the surface silicon layer 12 and the polysilicon gate layer 14 overlap and propagates in a direction perpendicular to the paper. When a voltage is applied between the region 212 of the surface silicon layer 12 and the region 214 of the polysilicon gate layer 14, free carriers (electrons or holes) are accumulated or depleted in the region 112 and the region 114 depending on the orientation of the voltage. The variation in the carrier density changes a refractive index by the carrier plasma effect, thereby modulating the phase of the light propagating through the region 101.